The invention relates to an infrared radiator having a heating element disposed in a quartz glass tube and a heating element containing carbon fibers, the ends of the heating element being connected to contact elements passing through the wall of the quartz glass tube. The invention furthermore relates to a method for the operation of such an infrared radiator.
Infrared radiators of the stated kind are disclosed, for example, in DE 198 39 457 A1. They have spiral-shaped heating elements of carbon fibers. Such carbon fibers have the advantage that they permit rapid temperature change, so they are characterized by great speed of reaction. Due to its spiral shape and the great surface area which it provides, the known carbon radiator has a relatively high radiation output and is suitable for operation at temperatures below 1000° C. In its practical form, heating element temperatures of maximum 950° C. are preferred. The achievable radiation power is limited by this top temperature limit.
Similar infrared radiators are described in DE 44 19 285 A1. Here a carbon ribbon is formed in a serpentine manner from a plurality of interconnected sections. GB 2,233,150 A likewise discloses infrared radiators in which the heating element is configured as a carbon ribbon. Infrared radiators with metallic heating elements are disclosed in DE-GM 1,969,200 and GB 1,261,748 and EP 163 348 A1. On account of a relatively small surface area, these also can achieve only limited radiation output. It is known especially from the last two disclosures named to configure the heating elements such that they are in contact with the surrounding quartz tube and are supported thereon.
It is a general problem with infrared radiators that quartz tubes easily recrystallize above about 1000° C., especially in case of contact, so that they become unusable.
The present invention is addressed to the problem of offering an improved infrared radiator, especially one with greater radiation output and long life, and to describe a method for its operation.
This problem is solved as to the infrared radiator in that the heating element is spaced away from the wall of the quartz glass tube and that the heating element is centered by spacers on the axis of the quartz glass tube, and nevertheless the spacers are heat bridges. Surprisingly it has been found that thus the temperature of the heating element can be increased substantially without recrystallizing the quartz glass tube, since the contact with the heating element (carbon fibers) causing the recrystallization is prevented. Especially it is advantageous for the achievement of a high radiation output if the heating element is in the form of a spiral or coiled ribbon.
It is appropriate that the inside diameter of the quartz glass tube be at least 1.5 times as great as the diameter of the spiral or coil of the heating element. At such a distance apart, preferably at such a diameter ratio, preferably at a ratio of about 1.7, the temperature of the heating element can be increased to definitely more than 1000° C. At a diameter ratio of about 2.5, the temperature of the heating element can be raised to temperatures above 1500° C., so that the radiation power, which is proportional to the fourth power of the absolute temperature, increases accordingly.
Advantageously, the spacers are made of molybdenum and/or tungsten and/or tantalum or of an alloy of at least two of these metals. It has been found that such spacers have on the one hand great thermal stability, but on the other hand the heating of the quartz glass tube to its recrystallization is prevented.
It is especially advantageous to a stable arrangement of the heating element that the spacers have at least at their side facing the heat elements, an expanse lengthwise of the heating element that is greater than the distances formed in this longitudinal direction between the coils of the heating element. Thus any slippage of the spacers into the gaps between the individual spirals is prevented even in the case of vibration.
It is appropriate to provide ceramic between the heating element and the spacers, especially aluminum oxide or zirconium dioxide, since this increases the life of the heating element and prevents premature burnout.
It is furthermore advantageous to make the contact elements of resilient material at their ends connected to the heating element, in order to assure reliable fixation of the contact elements before they are welded to additional contacts. Molybdenum can be used especially as resilient material.
The ends of the contact elements which are connected to the heating element can also be in the form of sleeves clutching these ends of the heating element; the sleeves can be made of molybdenum.
It has proven to be advantageous to provide graphite, especially graphite paper, between the ends of the heating element and the contact elements, in order to optimize the galvanic contact between the contact element and the carbon fibers of the heating element. The heating element appropriately consists substantially or exclusively of carbon fibers.
Between the graphite and the heating element, a noble metal paste and/or a metallic coating applied to the ends of the heating element can be provided. The metal coating can be formed of nickel or a noble metal and can preferably be applied galvanically.
Thus the contact is further improved. Welding of the contact-making parts can be done by resistance welding or laser welding.
The problem is solved for the method of operating an infrared radiator in that the heating element is heated to a temperature greater than 1000° C., preferably greater than 1500° C.